The invention relates to an exhaust gas purification system for an internal combustion engine of a motor vehicle.
DE 199 23 781 A1 discloses an exhaust gas purification system comprising a first particulate filter element and a second particulate filter element connected downstream thereof. The second particulate filter element thereby has a reduced transmission of soot particles compared to the first particulate filter element. Either an oxidation catalyst for oxidation of nitrogen monoxide (NO) to nitrogen dioxide (NO2) is assigned to the particulate filter elements, or they are respectively provided with a catalytic oxidation coating effective in that respect. Due to this embodiment, every particulate filter element is provided with sufficient NO2 for the low temperature oxidation of deposited soot.
Exemplary embodiments of the present invention provide an exhaust gas purification system for exhaust gas of internal combustion engines, the effectiveness of which for removing particles can be monitored in a reliable manner.
According to exemplary embodiments of the invention, charge sensor are provided for the exhaust gas purification system of the internal combustion machine mainly operated with air excess, which provide a magnitude correlating with a particle charge of the second particulate filter element. An evaluation unit assigned to the exhaust gas purification system is further provided, which evaluates the magnitude provided by the charge sensor in relation to a deposit effect of the first particulate filter element.
The first particulate filter element can have a high particle deposit degree of typically more than 99%, so that the exhaust gas has a very low particle content downstream of the intact first particulate filter element. During a malfunction of the first particulate filter element, for example due to a defect by crack formation, its filtering effect reduces and the exhaust gas flowing from the first particulate filter element contains added particles, but which are filtered by the second particulate filter element connected downstream. The second particulate filter element is thereby increasingly charged with particles, which can be sensed with the charge sensor provided according to the invention by providing a magnitude correlating with the particle charge of the second particulate filter element. The deposit effect of the first particulate filter element can be determined or estimated by the evaluation of the provided magnitude and a functional check or monitoring of the first particulate filter element can thereby take place. If, for example, a slow increase or a low extent of the particle charge of the second particulate filter element is determined by the charge sensor, the first particulate filter element is found to be functioning correctly. But if the charge increase of the second particulate filter element is faster than given by a defined comparison value, or if the particle charge of the second particulate filter element exceeds a given threshold, this is seen as a malfunction of the first particulate filter element and a corresponding message can displayed. The threshold can be preset, for example depending on a total operating time or an operating time since the last successful thermal regeneration of the first or the second particulate filter element.
In accordance with exemplary embodiments of the invention, the second particulate filter element is arranged in the exhaust gas purification system in such a manner that the heat amount transferred during a thermal regeneration of the first particulate filter element to the second particulate filter element is sufficient to heat the second particulate filter element to a temperature necessary for combusting deposited soot. A thermal regeneration in this context is a regeneration due to an oxygen-induced soot combustion. Depending on the soot composition, this takes place at temperatures of about 600° C., wherein a reduction to 500° C. can be achieved by employing catalytically acting additives or coatings.
A heat transfer from the first particulate filter element to the second particulate filter element thereby mainly takes place in a convective manner through the exhaust gas, which has been heated on its way to the first particulate filter element and/or through the first particulate filter element or which initially already has a high temperature. Additional heating taking place during a thermal regeneration of the first particulate filter element due to a heat release during the soot combustion can thereby also be used. The exhaust gas entering the second particulate filter element can thereby heat the second particulate filter element to or above a soot combustion temperature, so that it is regenerated by oxygen-induced soot combustion. For achieving a preferably aspired good heat transfer, it is thereby advantageous if the second particulate filter element is arranged near or immediately behind the first particulate filter element. An arrangement in the same housing can be employed. In this manner, a heat transfer by radiation can take place in addition to a convective heat transfer.
Due to the embodiment according to the invention, the second particulate filter element can compensate the loss of the filter effect especially with a fractionally deteriorated first particulate filter element with regard to the deposit degree. A high particle reduction is nevertheless achieved thereby, even with damaged first particulate filter element to a certain extent.
In a further aspect of the present invention, the particle capacity of the first particulate filter element is a multiple of the particle capacity of the second particulate filter element. The reduced particle capacity of the second particulate filter element compared to the first particulate filter element can be realized by designing it small compared to the first particulate filter element. The second particulate filter element can have less than about 50%, especially less than 30% of the volume of the first particulate filter element with an otherwise same design. With a comparatively low reduction of the deposit degree of the first particulate filter element, the load of the second particulate filter element is still relatively low, so that its purification effect for an altogether low particle emission is sufficient, especially as soot particles deposited in the second particulate filter element are combusted from time to time or depending on demand in connection with a thermal regeneration of the first particulate filter element. The second particulate filter element is also regenerated with this regeneration, and can again filter particles. A lower particle capacity of the second particulate filter element can however also be achieved by a lower porosity. The particulate filter elements are, however, designed the same in this respect, where a particulate filter with a flow-through wall can be employed. A surface filter, for example in the form of a sinter metal filter or depth filter, for example in the form of ceramic foam can alternatively be employed.
In accordance with another aspect of the present invention, the first and the second particulate filter elements are designed in such a manner that a reception of a defined particle amount at the second particulate filter element causes a significantly increased rise of the flow resistance compared to the first particulate filter element. Due to this design, a particularly sensitive functional monitoring of the first particulate filter element is enabled, as a small particle charge of for example 1 g/l or less already causes a significant increase of the flow resistance, whereas a particle charge of the same size with the first particulate filter element effects a low or negligible increase of the flow resistance. As a result, a malfunction or a reduced deposit degree of the first particulate filter element can be determined quickly by the charge sensor. In a further design of the invention, the charge sensor comprise back pressure sensor or flow resistance sensor. This forms a particularly robust and simultaneously economic embodiment for sensing the filter charge. A differential pressure sensor can be employed, whereby a reliable determination of a back pressure or flow resistance correlated with the charge is enabled.
In accordance with a further aspect of the present invention, the charge sensor comprise back pressure sensor or flow resistance sensor. As, with soot combustion during a thermal regeneration, a heat amount correlated with the amount of the deposited or combusted soot is released, a temperature increase of the exhaust gas or of the particulate filter element effected thereby can be evaluated with regard to the deposited or combusted soot amount. If the temperature increase exceeds a given value, this indicates an increased particle load of the second particulate filter element and thus a faulty first particulate filter element.